1. Field of the Invention
The present invention relates to a titanium alloy and a process for producing the same. Particularly, it relates to a noble β titanium alloy, which can offer wider utilization fields and applications, and to a process for producing the same.
2. Description of the Related Art
Titanium alloys are often used in the special fields such as aviation, military affairs, space, deep-sea exploration and chemical plants, because they are good in terms of the specific strength and corrosion resistance. In view of the structure, titanium alloys are classified as α alloys, α+β alloys, and β alloys. α+β titanium alloys, such as Ti-6% by mass Al-4% by mass V, have been often used so far. However, β titanium alloys have been attracting engineers' attention recently, because they are good in terms of the processability, strength and flexibility. In addition to the special fields, β titanium alloys are about to be used in more familiar fields, such as organism compatible products (for instance, artificial bones), accessories (for example, clocks or watches and frames of eyeglasses) and sporting goods (for instance, golf clubs), for example.
Incidentally, which phases titanium alloys form depends greatly on the type and content of containing alloying elements. For example, β titanium alloys are usually produced by including β phase stabilizing elements such as Mo in a relatively large content and thereafter carrying out solution treatments.
In the production of β titanium alloys, there are a variety of β phase stabilizing elements to be added. However, the stabilizing degree of β phase depends on the respective elements. Moreover, even in β titanium alloys, α phase stabilizing elements such as Al are often included in an appropriate content in order to improve the strength. Accordingly, it is very meaningful if an index is available, index that judges which titanium alloys are produced in dependent of the type and content of alloying elements to be included. The molybdenum equivalent “Moeq” is one of such indexes. The “Moeq” indexes the stability of β phase. When the “MOeq” is large sufficiently, the stability of β phase increases so that it is likely to produce β titanium alloys. On the contrary, when the “Moeq” is small, it is likely to produce a titanium alloys. Moreover, in the intermediate region, the resulting titanium alloys are likely to be α+β titanium alloys.
The following are literatures relating to titanium alloys: Japanese Unexamined Patent Publication (KOKAI) No. 8-224,327 (now issued as Japanese Patent No.2,999,387), Japanese Unexamined Patent Publication (KOKAI) No. 2000-204,425, Japanese Unexamined Patent Publication (KOKAI) No. 9-322,951, Japanese Unexamined Patent Publication (KOKAI) No. 7-292,429, Japanese Unexamined Patent Publication (KOKAI) No. 7-252,618, Japanese Unexamined Patent Publication (KOKAI) No. 9-209,099, Japanese Unexamined Patent Publication (KOKAI) No. 10-94,804, Japanese Unexamined Patent Publication (KOKAI) No. 10-265,876, Japanese Unexamined Patent Publication (KOKAI) No. 11-61,297, and Metallurgical Transactions A, vol. 19A, March 1998 pp. 527-542.
Among the literatures, the first four patent publications specify titanium alloys with the “MOeq.” For example, Japanese Unexamined Patent Publication (KOKAI) No. 8-224,327 discloses an α+β titanium alloy whose “Moeq” is from 2 to 10% by mass. Moreover, Japanese Unexamined Patent Publication (KOKAI) No. 2000-204,425 discloses an α+β titanium alloy whose “MOeq” is from 2 to 4.5% by mass. In addition, paragraphs [0014] and [0022] of Japanese Unexamined Patent Publication (KOKAI) No. 9-322,951 disclose an α+β titanium alloy whose “Moeq” is from 0 to 10% by mass. In the patent publications, though as comparative examples, there are descriptions to the effect that β equi-axis crystalline single phase is formed when a Ti-10% V-2% Fe-3% Al alloy whose “Moeq” is 9.5% by mass and a Ti-15% V-3% Al-3% Cr-3% Sn alloy whose “Moeq” is 11.5% by mass are quenched from the casting states. Note that all of the contents of the constituent elements are expressed in percentages by mass.
Paragraphs [0012] of Japanese Unexamined Patent Publication (KOKAI) No. 7-292,429 discloses a quasi-stable β titanium alloy which comprises Ti, Fe, Nb and Al and whose “Moeq” is greater than 16% by mass. Moreover, the patent publication discloses to the effect that a 100%-β structure is formed when the five titanium alloys whose “Moeq” is 11.5% by mass or more are quenched from the β transformation temperature or more.
However, note that the titanium alloys disclosed in all of the four patent publications include interstitial solution elements such as oxygen (O) in a content of less than 0.3% by mass.
On the other hand, the latter five patent publications disclose titanium alloys which include O and the like in a relatively large content. All of the titanium alloys disclosed in the latter five patent publications are α+β titanium alloys, or titanium alloys comprising α′ phase and β phase.
Moreover, the last literature discloses a Ti-2% by mass Al-16% by mass V-0.59% by mass O alloy. The “Moeq” and oxygen content of the titanium alloy is 8.7% by mass and 0.59% by mass, respectively. However, the aluminum content of the titanium alloy is so large as 2% by mass that the elastic deformability does not reach 1%. In addition, as can be seen from FIG. 15 of the literature, the titanium alloy is poor in terms of the ductility, and exhibits such a low tensile strength as less than 1,000 MPa.
It is pointed out herein that none of the literatures set forth actively and positively on the Young's modulus of titanium alloys.